1. Field of The Invention
The present invention relates to a process for reinforcing a semiconductor wafer in which a reinforcing member is adhered to the wafer with an adhesive, and particularly to a process for reinforcing a semiconductor wafer in which the adhesive is a hot-melt type and deaerated under a vacuum or reduced pressure conditions.
2. Related Background Art
A semiconductor wafer sliced off from a semiconductor crystal ingot is chemically or mechanically ground to be mirror-polished. Afterward, luminescent diodes, transistors and the like are formed on one of the faces of the wafer (i.e., the device-formed face) by an ordinary process, and then the wafer may sometimes be made thinner by grinding another face chemically or mechanically. Before applying the process, the wafer is reinforced or mechanically strengthened by making a reinforcing plate adhere to the device-formed face with a hot-melt adhesive.
It is important that no gas bubbles are included in the adhesive in this process of adherence of a reinforcing plate. The inclusion of bubbles causes the following troubles especially when the semiconductor wafer is thin and of a brittle material (for example, InP, GaAs and the like). First, gas bubbles in the hardened adhesive expand due to being heated as the wafer adhering to the reinforcing plate is ground by a grindstone. An expansion of the bubbles causes that part of the semiconductor wafer to bulge, and consequently that part is over-ground and the thickness may be uneven. A drawback brought about is that every semiconductor chip made through a following dicing process has a different thickness from each other. Second, gas bubbles expand as the wafer is treated in a vacuum vessel after it is adhered to a reinforcing plate and ground thinner, especially when the treatment accompanies heating. In that event, the wafer itself may be broken due to a local stress exerted by the expansion of the bubbles. The probability of the wafer being broken is high, especially when the part was over-ground to be thinner at a location of a bubble.
As an example of a technology to avoid this type of problem, there is one described in Japanese laid open Application No. 2-123726 by HAYASE et al. They propose that the ambient pressure be lowered to a vacuum before a semiconductor wafer is adhered closely to a reinforcing plate with an adhesive. With such a technology, even if some gas bubbles remain in the hardened adhesive between the wafer and the reinforcing plate which are closely in contact with each other, pressure in the bubbles is relatively low and an amount of expansion of the bubbles due to heating is relatively small, which little deforms the wafer locally. When a weight pushes down the reinforcing plate to the wafer, the lower the viscosity of the liquid adhesive is, the more easily the bubbles move in the adhesive, and consequently bubbles left in the hardened adhesive are small and few.
However, it is impossible to make the viscosity of an adhesive sufficiently low. Additionally, generation of a certain amount of bubbles cannot be avoided although the pressure in the bubbles is low. As a wafer is thinner, expansion of the bubbles deforms the wafer more, which makes the probability of breakage high. This problem occurs more easily especially when a reinforced wafer is exposed to a vacuum and to high temperature conditions during a following step. Additionally, if a reinforced semiconductor wafer is put in a wet treatment such as a rinse process, a rinsing liquid or etching liquid invades the bubbles, and the device-formed face is contaminated.
It is an object of the present invention to provide a process for reinforcing a semiconductor wafer in which the number of remaining or residual bubbles in a hot-melt adhesive can be reduced.
It is another object of the present invention to provide a process for reinforcing a semiconductor wafer in which a volume of each of the remaining or residual bubbles in a hot-melt adhesive can be reduced.
It is still another object of the present invention to provide a reinforced semiconductor wafer which does not develop a defective part even during a treatment under a vacuum, under heating, and under a dipping treatment in a liquid.